This invention relates to a vehicle traction control system and, more particularly to such a system which responds rapidly to excessive wheel spin during vehicle launch and which tightly controls wheel spin at high vehicle speeds.
It is a common experience with automotive vehicles for excessive wheel spin to occur during vehicle acceleration when the operator initiated engine torque delivered to the vehicle driven wheels are such that the frictional forces between the tire and the road surface are overcome. While a small amount of spin between the tire and road surface is necessary in order to achieve a driving force, excessive spin results in the reduction of effective driving force and in the deterioration in the lateral stability of the vehicle.
Various methods have been proposed for preventing an excessive spinning condition of the driven wheels of a vehicle. These methods include the adjustment of engine torque and/or the application of the brakes of the driven wheels when an excessive spinning condition is detected. Whichever method is selected for controlling acceleration wheel spin, it is desirable (A) to quickly establish control of spin if excessive spin should occur during launch of the vehicle from a standstill position to prevent large excursions of wheel spin and to maximize vehicle acceleration and (B) to control the acceleration wheel spin so as to continuously maintain lateral stability of the vehicle.
Various parameters have been proposed for controlling the engine torque output or the brakes of a spinning wheel in order to limit excessive spin. One such parameter is the difference between the speeds of the driven wheel and vehicle velocity as represented by the speed of an undriven wheel. However this speed difference may be very low at low vehicle speeds, such as during the launch phase of the vehicle, even though the driven wheel may be experiencing a very large spin ratio (the difference between the driven wheel speed and vehicle speed as represented by undriven wheel speeds, divided by the driven wheel speed). For example, at a vehicle speed of 3 miles per hour, a driven wheel may be traveling at a measured speed of 6 mph resulting in a difference in velocity of 3 mph but yet be experiencing a 50% spin ratio. The control of wheel spin in response to the difference between the driven wheel speed and vehicle speed at low vehicle speeds may result in slow response to an excessive spin condition resulting in a large excursion of spin of the driven wheels. On the other hand, at low vehicle speeds such as during the launch phase of the vehicle, control of wheel spin in response to the spin ratio would provide a rapid response to the excessive spin condition and provide improved vehicle acceleration while maintaining lateral stability.
However, if control of acceleration wheel spin were based upon the spin ratio at high vehicle speeds, the lateral stability of the vehicle would decrease since large differences between the driven wheel speed and vehicle speed may result even at low values of the spin ratio. For example, at a low vehicle speed of 10 mph, a 20% spin ratio exists when the speed differential between the driven wheel and the vehicle is 2.5 mph. However, at a vehicle speed of 40 mph, a 20% spin ratio exists when the speed differential between the driven wheel and the vehicle is 10 mph. At high vehicle speeds, control of wheel spin based upon spin ratio (as is desirable at low vehicle speeds), would allow large differential velocities resulting in a deterioration in the lateral stability of the vehicle. It can be seen that at high vehicle speeds, it would be more desirable to utilize the difference in the driven wheel speed and vehicle speed for control of the acceleration wheel spin. Such control would provide a tight limit on the wheel spin having the effect of maintaining the lateral stability of the vehicle while maximizing vehicle acceleration.